fix(ui): 拆分媒体骨架底色，恢复可见度

lib/pages/recommend/recommend_page.dart

@@ -426,7 +426,7 @@ class _RecommendFeedSkeleton extends StatelessWidget {
               height: 176,
               child: DecoratedBox(
                 decoration: BoxDecoration(
-                  color: AppColors.skeletonSurface,
+                  color: AppColors.skeletonMedia,
                   borderRadius: BorderRadius.all(Radius.circular(12)),
                 ),
               ),
@@ -465,7 +465,7 @@ class _SkeletonLine extends StatelessWidget {
       width: width,
       height: height,
       decoration: BoxDecoration(
-        color: AppColors.skeletonSurface,
+        color: AppColors.skeletonMedia,
         borderRadius: BorderRadius.circular(6),
       ),
     );